The present invention relates generally to integrated circuit (IC) devices, and more specifically, to frequency calibration of a ring oscillator that is performed in real-time during resistor trimming.
Many IC devices have timing requirements that require clock waveforms that are precisely synchronized with a reference clock waveform. A ring oscillator is one type of clock generator that is used to generate clock waveforms in an IC device. A ring oscillator generally includes a chain of N-stages of single-ended digital inverters, with the output of the last stage fed back to the input of the first stage. An output of the ring oscillator, which is obtained from an output node of an arbitrary inverter, oscillates between two limits, a logical high and a logical low.
The frequency of the oscillating output signal generated from the ring oscillator typically depends on the number of stages of inverters, the delay time of each stage, and a wiring delay which is determined by the equivalent output resistance and the input capacitance of successive stages. For example, a lower oscillation frequency can be obtained by increasing the number of stages of inverters and inserting a resistor between the successive inverters to form a RC delay unit. Nonetheless, the frequency of the output signal generated from a ring oscillator used in an IC device can shift from its intended operating frequency. This shift or variation in frequency can be due to process variations that occur during manufacturing of the IC device, or the variation can be caused by temperature and voltage variations that occur during operation of the device. A ring oscillator that experiences a wide range of operation frequencies can adversely affect the performance and reliability of an IC device that employs a ring oscillator. As a result, calibration of the ring oscillator is often necessary to minimize the wide range of operation frequencies that can adversely affect the performance and reliability of an IC device that deploys such an oscillator.